Sounds like you've narrowed it down! When it runs, does it run in gear without the terrible noises? If so, I would take out the starter motor and inspect it. I believe someone posted a really nice re-build video for the starter. I would remove it and start breaking it down to see what's making the grinding. There are some planetary gears that are a likely suspect.

One of the few problems with the xj 550/650/750 is the starter clutch slipping , lack of regular oil changes , using synthetic oil can all be symptoms. It can sound like a bunch of bolts in a coffee can when trying to start , mine slips sometimes when hot , only real fix is to split case and replace rollers and springs. Agree with dkavagh try to rebuild starter first , if brushes are worn and reduction gears need greased .

One of the few problems with the xj 550/650/750 is the starter clutch slipping , lack of regular oil changes , using synthetic oil can all be symptoms. It can sound like a bunch of bolts in a coffee can when trying to start , mine slips sometimes when hot , only real fix is to split case and replace rollers and springs. Agree with dkavagh try to rebuild starter first , if brushes are worn and reduction gears need greased .

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Ah man I hope it’s not the starter clutch. I did a little bit of research just now and that seems like a huge pain to fix.
I may try doing an oil change and use conventional oil.

Using synthetic oil can cause starter clutch to slip, the oil "washes" deposits , when we switched oil brands in the aircraft , we had oil filter clogging problems so bad we switched to yet another brand . Some folks claim they have ran synthetic oil with no problems , but the tollarances on these engines are not as "tight" on modern engines . I used Screaming Eagle 20/50 w synthetic in my Buell ,but it had a dry clutch .

I got the battery tested, it's fine.
I removed the starter and opened it up. Lots of carbon buildup and muck inside it. Started cleaning.

Here's some pictures of the brushes and various components.

tested continuity from brushes to cable terminals and cable terminals to casing and I think, going by the Clymber Manual, that's okay.

checked resistance on component in the first picture, forgot what it's called, but used the Clymber Manual and the readings were good. Also checked the mica levels between the copper panels. Also good. So whatever that component is called, I think it's okay. Again, this is the all from the first picture.

I need to go to the store tomorrow and get an electrical device cleaning solvent but judging by these pictures, can you guys see anything major wrong? Again, whatever needed to be tested with an ohmmeter, I did that and results seem to be fine. Maybe a starter motor rebuild kit is in order, but before I do that I'd like to see what you all think.

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